1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device and a driving method thereof.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Much attention has been focused on a semiconductor device that retains data by using a combination of a transistor in which silicon (Si) is used for a semiconductor layer and a transistor in which an oxide semiconductor (OS) is used for a semiconductor layer (hereinafter “OS transistor”) (see Patent Document 1).
In recent years, with the increase in the amount of data manipulated, a semiconductor device having a large storage capacity has been required. In such situations, the semiconductor device disclosed in Patent Document 1 has a structure in which multilevel data is stored and read.